The present application relates generally to controlling a compressor recirculation valve in a vehicle fuel cell system, and more particularly to systems and methods for controlling a compressor recirculation valve to help meet a fuel cell stack cathode flow setpoint, especially during transient operating conditions.
Automotive technology is rapidly expanding in the area of finding alternatives to using gasoline as the primary source of energy in vehicle propulsion systems. One area of interest in recent years has focused on utilizing gaseous reactants as fuel. Gases, such as hydrogen, provide a promising alternative to gasoline because of their abundance in nature. In some vehicular systems, propulsion may be achieved by using hydrogen gas as part of a chemical reaction within a fuel cell to generate electrical energy that can be used to power an electric motor. The generated electricity can then be used to propel the vehicle through the motor, either alone or in conjunction with a petroleum-based combustion engine. Such fuel systems also typically produce less pollution than petroleum-based ones.
In a typical fuel cell, hydrogen or another reactant gas is supplied to the anode of the fuel cell, where the hydrogen is broken into electrons and positively charged ions. An electrolyte layer separates the anode from the cathode, allowing the ions to pass to the cathode, while preventing electrons from passing to the cathode. Instead, electrons are routed around the electrolyte layer through a load and back to the cathode, allowing electrical power to be harnessed. At the cathode, the ions, electrons, and supplied oxygen or air are typically combined to produce water and heat. Individual fuel cells may be arranged in series or parallel as a fuel cell stack in order to produce a higher voltage or current yield. Furthermore, still higher yields may be achieved by combining more than one stack.
In a vehicle utilizing fuel cell technology, these reactant gases may be transported and used within a pressurized gas system. For example, stored hydrogen may be provided to a fuel cell anode and chemically reacted to generate an electrical current. Similarly, air may be received by an air intake and provided to the cathode of a fuel cell. Such vehicle fuel cell systems require the use of compressors to perform such pressurizing function, and may additionally include ancillary equipment such as valves, controllers or the like to regulate the flow of a reactant gas between the compressor and fuel cell.
An inherent attribute of such compressors (at least as they relate to cathode-side operation) is that the cathode's pressure control and flow control are coupled together. Such coupling tends to destabilize system operation, especially during periods of transient system operation. These concerns are particularly acute during transient operating conditions, where both the flow setpoint and the pressure setpoint may exhibit near-instantaneous changes. Even more particularly, the difficulties of ensuring proper recirculation flow are especially acute during downtransient operation, as the inertial effects of a compressor being asked to slow down prevent the compressor speed from dropping as fast as needed; this in turn has a propensity for causing a recirculation flow mismatch. Because the water production in a fuel cell stack varies with current, a downtransient event (where the stack current is suddenly reduced) causes a concomitant rapid drop of the water production. If stack cathode air flow is not reduced along with this current reduction, the stack will quickly dry out, resulting in damage to it. Likewise, the mismatch can also be due to not enough flow being delivered to the cathode. In this case, the presence of a recirculation valve may cause too much recirculation may take place, which results in stack flooding. As such, merely having a recirculation valve does not—in and of itself—ensure quick, precise control to avoid both stack overflow dryout and stack underflow flooding.